Hello Laurenz,
The above metric works fine for my primary server. However, We have a cascading setup in our production system. This particular query doesn't work for my intermediate server which is standby to the primary server but also a master to one more standby server. We get the following error:
prod=# SELECT round(pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn) / (1024 :: NUMERIC ^ 2), 2 ) AS replay_lag FROM pg_stat_replication where application_name like 'rtv%';
ERROR: recovery is in progress
HINT: WAL control functions cannot be executed during recovery.
I wanted to also track the transfer/replay lag between the intermediate server and the final standby. I narrowed down the problem with pg_current_wal_lsn() procedure. Any idea on how to mitigate this? or any alternative to the pg_current_wal_lsn() procedure that I can use to get transfer lag?
Thanks,
Hello Laurenz,
Thank you so much for sending the query. It was exactly what I needed. I just made 1 modification to beautify the transfer and replay lag and I can see the size in bytes.
SELECT application_name, pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), flush_lsn)) AS transfer_lag,
pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn)) AS replay_lag
FROM pg_stat_replication;
I am now using zabbix to constantly monitor them and notify myself if it breaches a certain threshold.
Thanks again!
Best,
Viral Shah
Nodal Exchange LLC
On Wed, 2021-04-14 at 17:50 -0400, Viral Shah wrote:
> We have a PostgreSQL 10.12 cluster of servers in two different data centers.
> Off lately, in the case of a large WAL generation, we are seeing replication
> delay between the master and the standby server. These delays have off lately
> been there for an unusually long time. I was wondering if we have any metric
> that can calculate the amount (size) of WAL transfer left between master and
> standby?
>
> PS: We have ensured we have upgraded our firewalls for better speed transfer.
>
> Any help on how to figure out the slowness in the WAL transfer would be much appreciated.
SELECT pg_wal_lsn_diff(pg_current_wal_lsn(), flush_lsn) AS transfer_lag,
pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn) AS replay_lag
FROM pg_stat_replication;
If both are delayed, it might be that the network cannot cope.
If only the second number is delayed, you have replication conflicts
with queries on the standby.
Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com
